This invention generally relates to coatings suitable for protecting components exposed to high-temperature environments, such as the hot gas flow path through a gas turbine engine. More particularly, this invention is directed to coatings that impart oxidation resistance to niobium-based materials, and particularly monolithic niobium-based alloys.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. While nickel, cobalt and iron-based superalloys have found wide use for components within the hot sections of gas turbine engines, alternative materials have been proposed to achieve improved high-temperature properties. For example, refractory metal alloys and intermetallic materials have been proposed as candidates for high temperature applications, such as blades, nozzles, vanes, shrouds, afterburner components, and other hot gas path components of gas turbine engines. Examples include monolithic refractory metal alloys and refractory metal intermetallic composite (RMIC) materials, including alloys based on niobium, titanium, hafnium and zirconium. A notable example of an RMIC is niobium silicide (Nb—Si) alloys, which may further contain other alloying constituents, including titanium, hafnium, aluminum, chromium, etc. Nb-based RMICs usually have a microstructure containing a metallic Nb-based phase and one or more intermetallic phases, for example, an intermetallic niobium-silicide (NbSi) phase that forms in-situ during solidification of the alloy. The niobium-based phase is relatively ductile, while the intermetallic phase is brittle and stronger to contribute strength, fracture toughness and other desirable mechanical properties. In contrast to niobium silicide alloys and other Nb-based RMICs, monolithic Nb-based alloys (as the term is used herein) are essentially free of strengthening phases such as silicides and other intermetallic phases.
The above-noted compositions are termed Nb-based in that their predominant constituent is niobium; in other words, the material contains more niobium than any other individual constituent. Depending on the particular composition, niobium-based alloys and RMICs can have much greater temperature capabilities than existing nickel, cobalt, and iron-based superalloys.
Nb-based RMICs and monolithic Nb-based alloys have become of particular interest for replacing superalloys in the hot sections of turbine engines because they are capable of possessing a promising combination of low-temperature mechanical properties, such as room temperature toughness, as well as high-temperature strength and creep resistance. However, a significant impediment to the development of Nb-based alloys and RMICs for high-temperature applications in aircraft engines is their oxidation resistance. Existing nickel, cobalt, and iron-based superalloys exposed to high-temperature oxidation environments are often protected with an oxidation-resistant coating that forms a protective oxide film during high temperature exposure in air. The oxide film, commonly referred to as an oxide scale and often an aluminum oxide (alumina, Al2O3) or chromium oxide (chromia, Cr2O3), inhibits oxidation of the coating and the underlying substrate. Nb-based materials intended for use in high-temperature oxidizing environments have been protected with oxidation-resistant coatings, for example, Si—Cr—Ti and Si—Cr—Fe silicide coatings, to improve their oxidation resistance. These coatings are interdiffused and form a large diffusion zone. The oxide scale formed by these coating compositions is a complex oxide that does not always exhibit a slow parabolic mass gain with time, represented by the equation:(Δm)2=kpt where Δm is the specific weight increase, t is time, and kp is a kinetic constant (always positive).
In an effort to provide improved coatings that exhibit a lower oxide growth rate, a significant body of work has been directed at using aluminum interdiffusion to produce a Nb—Al compound capable of forming a protective alumina scale. However, the sustained formation of alumina scale has not been achieved in these attempts, and to date aluminum-based coatings suitable for protecting Nb alloys has not been practical.
Another result of the interdiffusion process used to form Si—Cr—Ti or Si—Cr—Fe coatings is that a significant number of vertical cracks form in the coating, often in excess of one hundred vertical cracks per inch, when formed on Nb—Si composites. These cracks are termed “vertical” because they extend through the interdiffused coating in a direction that is roughly perpendicular to the coating/substrate interface. FIG. 3 shows three cross-sections of a Nb-based substrate coated with a Si—Cr—Fe coating commercially known as R512E (by weight, 60Si-20Cr-20Fe), in which cracks are visible and extend from the coating surface all the way into the substrate material, creating a path through which the substrate material is directly exposed to the oxidative environment. Oxides that form in the vertical cracks during high temperature exposure cause the cracks to wedge open, increasing the depth of the cracks into the substrate. As also shown in FIG. 3, with sufficient exposure time the cracks may be deflected at the coating/substrate interface and run along the interface, producing a wedging effect between the coating and substrate. Ultimately, the coating spalls because the crack-opening effect caused by oxidation exposes the substrate material directly to the oxidative environment.
Vertical cracks in Si—Cr—Ti or Si—Cr—Fe coatings are due in part to the interdiffused coating having a significantly larger coefficient of thermal expansion (CTE) than the substrate material, for example, 50% higher or more. The high CTEs of the Si—Cr—Ti and Si—Cr—Fe coatings are attributable to the silicides that form during the interdiffusion process, for example, Ti5Si3 (CTE of about 11.5 ppm/° C.), TiSi2 (CTE of about 12 ppm/° C.), and CrSi2 (CTE of about 14 ppm/° C.), whose CTEs are significantly greater than that of Nb—Si materials. A CTE difference between the coating and substrate of less than 10% would be desirable for reducing and potentially eliminating vertical crack formation.
In view of the above, coating compositions and methods are desired for coatings capable of improving the high temperature performance of Nb-based materials.